1. Field of the Invention
The present invention generally relates to methods and systems for analyzing a specimen using atomic force microscopy profiling in combination with an optical technique. Certain embodiments relate to systems and methods for positioning an atomic force microscope probe based on data generated by an optical subsystem.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a specimen such as a semiconductor wafer using a number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that typically involves transferring a pattern to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a semiconductor wafer and then separated into individual semiconductor devices.
As the dimensions of advanced semiconductor devices continue to shrink, metrology and inspection processes used to monitor and control semiconductor fabrication processes are becoming increasingly important and increasingly difficult. For example, in the past, critical dimension (CD) metrology was mostly performed with optical or electron microscopes. However, as the aspect ratio, which is generally defined as the height of a feature divided by its width, of integrated circuit features increases, measuring characteristics of the features in all three dimensions becomes more important and more difficult. Scanning probe microscopes (SPMs) are an attractive alternative to optical and electron microscopes for three-dimensional imaging of integrated circuit features. In addition, many SPMs are capable of atomic-scale resolution for a wide range of materials, even in ambient conditions.
Unlike optical and electron microscopes, SPMs do not use optics or electrons to obtain images. Instead, SPMs use a solid body (i.e., a probe tip) to perform measurements. In general, the probe tip is brought into close proximity with a specimen, and the probe tip is scanned back and forth in a raster fashion. The probe traverses the specimen at a constant distance from the specimen. By monitoring the displacement of the probe during scanning, a three-dimensional image of the specimen may be obtained. In this manner, a high resolution image of the specimen in all three dimensions may be achieved. An additional advantage of SPMs is that specimens can be imaged in a non-destructive manner.
One example of an SPM is an atomic force microscope (AFM). Force sensors are the most commonly used type of sensors for detection of the proximity of the probe tip to the specimen surface. For example, semiconductor manufacturing specimens often have poorly conducting or poorly insulating properties. In addition, force sensors are able to profile all regions of such specimens uniformly. Therefore, one main advantage of an AFM is that insulating samples as well as conductive samples may be imaged.
There are, however, some disadvantages to an AFM. For example, an AFM generally has a relatively low throughput. The relatively low throughput is due, at least in part, to the need to protect the probe tip from damage. Therefore, the AFM probe may not be scanned over the surface faster than its ability to respond to changes in surface height to avoid collisions between the AFM probe tip and the specimen surface. The throughput is low enough to prevent an AFM from being used as an in-line metrology tool for semiconductor device manufacturing. In addition, optical and electron microscopes are much faster than AFMs making these microscopes the metrology choice for in-line measurements.
In addition to slow imaging times, finding the features to be imaged with an AFM is also relatively time consuming. For example, the physical structure of most probe microscopes excludes high-magnification viewing of the probe tip on the specimen surface. In addition, micro-cantilever based AFMs may physically prevent the surface region of interest from being viewed. Therefore, the probe tip cannot be imaged near the surface region of interest, and surface features of interest must be found by imaging with the probe tip at relatively slow speed with a limited field of view. In most instances, the majority of probe tip wear is encountered during this imaging step.
Accordingly, it may be advantageous to increase the throughput of SPMs, and in particular AFMs, by decreasing the time required to position the probe tip prior to scanning thereby reducing damage caused to the probe tip during positioning of the AFM and improving the suitability of such microscopes for use as in-line measurement tools.